Specialized markets provide continuous impetus for the development of high performance pressure vessels. Performance is measured across a set of desirable attributes which differ in priority depending on the vessel's intended application. Two commonly identified primary performance attributes are robustness and high strength-to-weight ratio. Vessels engineered for operation in environments of lower pressure than required within the vessel can take advantage of the tensile stability offered by a flexible structure—with the conspicuous benefit, amongst others, of potential collapsibility and superior strength-to-weight ratio. The benefits derived from flexible architecture become more significant in operational environments of diminished gravitational force and especially in the vacuum of deep space.
Considerable interest in flexible pressure vessels therefore exists in the field of space exploration. This interest has been bolstered by the inescapable reality that the present launch cost for payload to Low Earth Orbit is high. A very rough yet optimistic ballpark estimate places launch costs at about US$10,000 per kilogram and almost $1 million per cubic meter. Consequently a pressure vessel which can be launched from Earth aboard a vehicle commensurate in size to the vessel's collapsed volume, to a destination in space where the vessel is subsequently deployed, will provide significant savings related to launch costs. A third primary performance attribute is a high ‘deployment efficiency ratio’ whereby the ratio of internal volume presented by the vessel after deployment is maximized with respect to the volume occupied by vessel when collapsed for packaging and transport. Flexible vessel structures used in space exploration demand the adequate fulfillment of the three performance attributes mentioned: robustness, high-strength-to-weight ratio, and high deployment efficiency ratio.
With the advent of a new generation of fiber technology in the 1990's, exemplified by the liquid crystal polymer Vectran® of Kuraray Co., Ltd., and PBO (Phenylene Benzobisoxasole), designers have been able to push the specific strength envelope far back while simultaneously broadening the spectrum of operable space environments for their architecture. Two dimensional gores can be cut from planar sheets of material to subsequently be connected edge to edge to reproduce a close facsimile of a fully closed three dimensional shell. A breakthrough that helped usher in the current era of ‘soft systems engineering’ was the successful application of airbags made from Vectran® fabric to cushion the 1997 NASA Pathfinder mission's landing on Mars. More recently, the same technology was used on a larger scale to safely land the beloved twin Rovers, Spirit and Opportunity, on Mars.
Fabric Gore Vessels
A representative example of current collapsible flexible pressure vessel art is the airbag system which cushioned the landing of NASA's exploration rovers on Mars: Built of woven fabric gores, the airbags were engineered for approximately 35 kilograms per centimeter membrane stress while needing to be of extremely high strength-to-weight construction, packaged very tightly, and exposed to a great variety of environmental extrema.
In general, only products comprised of high modulus fibers are considered viable for construction of load bearing components of high performance flexible pressure vessels since these fibers exhibit a tensile strength typically two to four times greater than nylon or polyester. While low elongation is a great benefit for maintaining vessel geometry, it becomes an Achilles' heel if the vessel is not meticulously engineered: Due to their high modulus it is difficult to guarantee proper load sharing between individual fibers within a woven fiber product, thereby making any interface, joint, or seam the potential point load induced source of chain reaction failure. This does not ultimately bode well for ‘broad’ fabrics since, the greater the width of the weave, the more difficult it becomes to precisely balance the load sharing between individual fibers.
The ultimate strength limitation which precludes use of fabric gore vessels in high load applications stems from the aforementioned inefficiency of fiber load sharing in broad fabrics combined with the necessity that individual meridional gores must be cut in a lens shape to allow for the assembly of the intended three dimensional pressure shell. When such a tapered gore profile is cut from a planar fabric, the load bearing fibers corresponding to the vessel shell's circumferential stress direction are severed, as are the meridional fibers where they intersect the gore edges converging on the poles. Consequently, the degree of preservation of the structural integrity of the vessel's shell relies almost entirely on how well load carrying pathways between adjacent gores are maintained across the meridional seams connecting the gores. Unfortunately, broad fabric cross-seam strength loss is always substantial, often over 40% for high performance fabric woven of high modulus fiber. Moreover, if higher membrane stresses require application of thicker fabric, the seam will carry a yet lower percentage of the base fabric strength due to the decreased load sharing precision amongst the greater number of fibers. Multiple layers of fabric are sometimes used to avoid thick mono-layers, however load sharing between fabrics constructed of high modulus fiber becomes not only an immense integration challenge but a nightmare due to rampant manifestation of indeterminate load pathways. A further limitation posed by heavier fabric structures is the increased seam sewing difficulty, most dramatically manifesting itself in the polar areas of meridional seam convergence and often the ultimate limiting factor in fabric gore structure design performance. Finding a solution to the meridional bulk convergence problem is one of the great recurring challenges in flexible vessel design.
Hybrid Vessels
Hybrid vessels comprise a substantially impervious barrier structure confined by an open grid of meridional and/or circumferential tendons made of webbing or cordage. Hybrid design is based on the premise that materials providing the surface coverage and impermeability necessary for the containment of the vessel's fluid contents are not ideal for simultaneously bearing the vessel's global pressure and mass loads and vice versa. Segregating material roles provides vastly greater design flexibility allowing the structure to be much more precisely tailored to application demands. Furthermore, the replacement of a single specialized ‘do it all’ material by a variety of materials chosen to each perform a specific function facilitates off-the-shelf component availability.
The barrier structure of the hybrid vessel is prepared oversized or with sufficient elasticity with respect to the restraining grid of tendons such that the vessel's global pressure confining stresses are carried by the restraint, while the barrier carries only the local pressure induced stresses generated where the barrier bulges outwards between restraint tendons. This approach to flexible vessel design opens the door to the capability of higher strength-to-weight efficiency and can be effectively tailored to very specific applications.
The drawback of hybrid vessels is the fact that no convenient solution has been found to maintain circumferential tendon hoops correctly positioned on the vessel's steep end cap surfaces. The end cap is an axially terminal end-closing structure of a pressure vessel. The problem has generally been circumvented by replacing what would have ideally been flexible end caps with rigid end plates or caps akin to those used to close the ends of metal or composite pressure vessels.
Natural Shape Hybrid Vessels—the Vessel of the Current Invention
As evidenced above, a cornerstone challenge in flexible vessel design is the development of correspondingly flexible end caps. In 1919, Sir Geoffery Taylor found when he reinforced a rubber weather balloon with meridional cords the balloon assumed a peculiar, oblate spheroidal, axial profile. He ultimately applied calculations correlating this geometry to characterize the shape assumed by certain descending parachutes [1].
This is significant because almost unwittingly a viable end cap is presented through simple, although perhaps counterintuitive, elimination of the entire circumferential portion of a hybrid vessel's restraint structure. The requirement for obtaining this default shape is simply the provision of a vessel with a restraint of meridional tendons and that there is sufficient excess barrier structure material to allow the barrier to form meridional lobes bulging outwards between the meridional tendons thereby precluding the barrier's carriage of any of the global circumferential stress of the vessel. The resulting geometry is referred to as the ‘natural shape’ of a flexible vessel and can be described as the geometry of equilibrium found when, through elimination of circumferential stress, the global pressure confining stress of the vessel is carried only by the meridional tendons.
While the calculational theory behind the natural shape was sporadically revisited during the five decades following Taylor's observations, notably by Upson [2], no commercial impetus for its application appeared on the horizon until modern materials such as Mylar® and Kevlar® of E.I. du Pont de Nemours and Company supported scientific and strategic interest in exploration of the upper atmosphere [3]. The only significant application of natural shape vessel design to date has been in the realm of giant high altitude balloons. With respect to the field of the current invention there are two drawbacks presented by the aforementioned balloon technology in general. Firstly the capability of these vessels is limited to extremely thin, low pressure membranes, and secondly, there is no provision for a vessel specifically intended to present both collapsed and deployed configurational functionality.